(1) Blood-brain barrier vessels (BV) can be converted ot permeable, fenestrated vessels (FV) in vivo by chemical means. Phorbol ester retinoic acid infused into rats' cerebral cortex convert about 30% of into the brain continuously for indefinite periods. (2) There is a short fetal period when skeletal muscle grafted to mature choroid plexus is supplied by FV as well as the expected continuous muscle type (CV). It is hypothesized that a conversion factor, changing CV to FV, is secreted by E14 but not E16 muscle. The grafts' FV are derived from choroid plexus, whose vessels became labeled with 3H-thymidine; but no graft vessels were tagged, maybe because tracer had not been injected more often. Vascular endothelial growth factor is not the converter because neither the factor itself nor its mRNA, were in the grafts. (3) The to La3+ with RMP-7, an analog of bradykinin. RMP-7 acts by, presumably, binding to its endothelial B-2 receptor with the result that endothelial tight junctions become permeable, allowing La3+ to pass through to them into the brain.